Plasma-enhanced substrate processing method and apparatus

ABSTRACT

A method for processing a substrate in a capacitively-coupled plasma processing system having a plasma processing chamber and at least an upper electrode and a lower electrode. The substrate is disposed on the lower electrode during plasma processing. The method includes providing at least a first RF signal, which has a first RF frequency, to the lower electrode. The first RF signal couples with a plasma in the plasma processing chamber, thereby inducing an induced RF signal on the upper electrode. The method also includes providing a second RF signal to the upper electrode. The second RF signal also has the first RF frequency. A phase of the second RF signal is offset from a phase of the first RF signal by a value that is less than 10%. The method further includes processing the substrate while the second RF signal is provided to the upper electrode.

PRIORITY CLAIM

The present application is a divisional application of and claimspriority under 35 USC 120 to a commonly assigned, previously filedpatent application entitled “Plasma-Enhanced Processing Method andApparatus”, application Ser. No. 11/618,583, filed on Dec. 29, 2006,issued Sep. 11, 2012 as U.S. Pat. No. 8,262,847, by the same inventorsherein, which is incorporated herein by reference.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is related to the following applications, all ofwhich are incorporated herein by reference:

Commonly assigned application entitled “Method and Apparatus forProcessing a Substrate Using Plasma”, filed on even date herewith by thesame inventors herein.

BACKGROUND OF THE INVENTION

In the processing of semiconductor substrates, plasma processing isoften employed. Plasma processing may involve differentplasma-generating technologies, for example, inductively-coupled plasmaprocessing systems, capacitively-coupled plasma processing systems,microwave-generated plasma processing systems, and the like.Manufacturers often employ capacitively-coupled plasma processingsystems in processes that involve the etching of materials using a photoresist mask.

Important consideration for plasma processing of substrates involves ahigh etch rate and a high photo resist selectivity. A high etch raterefers to the rate at which the target material is etched during plasmaprocessing. Generally speaking, the faster the underlying layer can beetched, a greater number of wafers can be processed per unit of time.All things being equal, manufacturers desire to process more wafers perunit of time to increase wafer processing efficiency. Photo resistselectivity refers to the discrimination between the photo resist maskand the underlying target layer during etching.

As circuit density increases, manufacturers are required to etch or toform a greater number of devices per unit area on the water. The higherdevice density requires a thinner photo resist layer. The thinner photoresist layer, in turn, tends to be more susceptible to beinginadvertently etched away. As a result, manufacturers constantly striveto create processing recipes that can etch the underlying layer at ahigh etch rate while avoiding damage to the photo resist mask.

One way to increase the etch rate is to increase the plasma densityduring plasma processing. In a capacitively-coupled plasma processingsystem, plasma density may be increased by increasing the power of thehigher frequency RF signals. To facilitate discussion. FIG. 1 shows aprior art multi-frequency capacitively-coupled plasma processing system100, the plasma processing system typically employed to processsubstrates. As seen in FIG. 1, multi-frequency capacitively-coupledplasma processing system 100 includes a chamber 102 which is disposed inbetween an tipper electrode 104 and a lower electrode 106.

In the implementation of FIG. 1, lower electrode 106 is provided withmultiple RF frequencies, such as 2 Megahertz, 27 Megahertz, and 60Megahertz. Upper electrode 104 is grounded in the implementation ofFIG. 1. Multi-frequency capacitively-coupled plasma processing system100 also includes a plurality of confinement rings 108A, 108B, 108C, and108D. The confinement rings 108A-108D function to confine the plasmawithin chamber 102 during plasma processing.

There is also shown in FIG. 1 a peripheral RF grounded ring 110,representing the RF ground for the plasma generated within chamber 102.To isolate peripheral RF ground 110 from upper electrode 104, aninsulating ring 112 is typically provided. A similar insulating ring 114is also provided to insulate lower electrode 106 from an RF ground 116.During plasma processing, the RF power provided to lower electrode 106excites etching gas provided into chamber 102, thereby generating aplasma within chamber 102 to etch a substrate that is typically disposedon lower electrode 106 (substrate is not shown to simplify FIG. 1).

As discussed earlier, it is highly desirable to etch the target layer onthe substrate while the substrate is disposed in chamber 102 withoutunduly damaging the overlying photo resist mask. In the prior art,increasing the etch rate of the target layer may be achieved byincreasing the plasma density within chamber 102. Generally speaking,the plasma density may be increased by increasing the power level of thehigher frequency RF signals that are provided to lower electrode 106 inthe context of the present invention, a high frequency RF signal isdefined as signals having a frequency higher than about 10 Megahertz.Conversely, RF signals with frequencies below 10 Megahertz are referredto herein as Low Frequency Signals.

However, by increasing the power level of the higher frequency RFsignals the 27 Megahertz, RF signal or the 60 Megahertz RF signal ofFIG. 1), it may be challenging to confine the generated plasma withinchamber 102. Even if the plasma can be satisfactorily confined, electronloss to the upper electrode during, plasma processing places an upperlimit on the plasma density within chamber 102. It has been found thatas the plasma density increases; electrons are lost to the groundedupper electrodes or other grounded surfaces of the multi-frequencycapacitively-coupled plasma processing system 100, thereby causing theplasma density within chamber 102 to reach a saturation point. Beyondthis saturation point, increasing the RF power of the higher frequencyRF signal does not increase the plasma density since the electron lossoutpaces the generation of ions.

Furthermore, increasing the RF power to the higher frequency RF signalshas been found to adversely affect the photo resist selectivity. At ahigh RF power level, the photo resist mask is damaged to a greaterextent due to increased bombardment, which causes the photo resist maskto erode away at a faster rate, thereby negatively impacting the etchingprocess.

FIG. 2 shows a prior art implementation whereby one or more highfrequency RF signals 202 (e.g., the 60 Megahertz RF signal of FIG. 2)are provided to upper electrode 104 in order to provide additionalcontrol over the generation of ions within chamber 102. However, theimplementation of FIG. 2 still does not solve the aforementioned problemof plasma density saturation point effect. When the RF power level ofthe higher frequency signal provided to upper electrode 104 isincreased, the aforementioned saturation point effect is also observed,limiting the plasma density and consequently, the etch rate through thetarget layer irrespective of the increase in the RF power level to thehigher frequency RF signals.

Additionally, other prior art implementation has tried to control thephoto resist selectivity by controlling the temperature of theelectrodes, it has been found that the approach of controlling thetemperature of the electrodes is minimally effective in controlling thephoto resist selectivity. Furthermore, the approach of controlling thetemperature of the electrodes does not address the aforementionedproblem of plasma density saturation point effect.

Therefore, various aforementioned prior art implementations have provenineffective in increasing etch rate without adversely affecting ormaintaining high photo resist selectivity in capacitively-coupled plasmaprocessing system in processes that involve the etching of materialsusing a photo resist mask. In the prior art implementation of FIG. 1,the increase in RF power to the lower electrode may lead tounconfinement of plasma, saturation point of plasma density, andadversely affect the photo resist selectivity. Whereas in the prior artimplementation of FIG. 2, the increase in RF power to the upperelectrode may lead to saturation point of plasma density. Furthermore,the prior art implementation of controlling temperature of theelectrodes is minimally effective in controlling the photo resistselectivity while providing no solution for the plasma densitysaturation effect.

SUMMARY OF INVENTION

The invention relates, in an embodiment, to a method for processing asubstrate in a capacitively-coupled plasma processing system, which hasas plasma processing chamber and at least an upper electrode and a lowerelectrode. The substrate is disposed on the lower electrode duringplasma processing. The method includes providing at least a first RFsignal to the lower electrode. The first RF signal has a first RFfrequency. The first RF signal couples with a plasma in the plasmaprocessing chamber, thereby inducing an induced. RF signal on the upperelectrode. The method also includes providing a second RF signal to theupper electrode. The second RF signal also has the first RF frequency. Aphase of the second RF signal is offset from a phase of the first RFsignal by a value that is less than 10%. The method further includesprocessing the substrate while the second RF signal is provided to theupper electrode.

The above summary relates to only one of the many embodiments of theinvention disclosed herein and is not intended to limit the scope of theinvention, which is set forth in the claims herein. These and otherfeatures of the present invention will be described in more detail belowin the detailed description of the invention and in conjunction with thefollowing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 shows a prior art multi-frequency capacitively-coupled plasmaprocessing system, representing the plasma processing system typicallyemployed to process substrates.

FIG. 2 shows a prior art implementation whereby one or more highfrequency RF signals are provided to upper electrode in order to provideadditional control over the generation of ions within chamber.

FIG. 3 shows a simplified diagram of an implementation wherein amirroring circuit is employed to detect an RF signal from the lowerelectrode and to provide the upper electrode with a transformed RFsignal that is in-phase with the RF signal of lower electrode duringplasma processing.

FIG. 4 a shows an example plot of an RF signal from the lower electrode.

FIG. 4 b shows an example plot of an RF signal directed to the upperelectrode running in phase with the RF signal from the lower electrodeof FIG. 4 a

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention will now be described in detail with reference toa few embodiments thereof as illustrated in the accompanying drawings.In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one skilled in the art, that the presentinvention may be practiced without some or all of these specificdetails. In other instances, well known process steps and/or structureshave not been described in detail in order to not unnecessarily obscurethe present invention.

Various embodiments are described herein below, including methods andtechniques. It should be kept in mind that the invention might alsocover articles of manufacture that includes a computer readable mediumon which computer-readable instructions for carrying out embodiments ofthe inventive technique are stored. The computer readable medium mayinclude, for example, semiconductor, magnetic, opto-magnetic, optical,or other forms of computer readable medium for storing computer readablecode. Further, the invention may also cover apparatuses for practicingembodiments of the invention. Such apparatus may include circuits,dedicated and/or programmable, to carry out tasks pertaining toembodiments of the invention. Examples of such apparatus include ageneral-purpose computer and/or a dedicated computing, device whenappropriately programmed and may include a combination of acomputer/computing device and dedicated/programmable circuits adaptedfor the various tasks pertaining to embodiments of the invention.

In accordance with embodiments of the invention, there are providedmethods and arrangements for controlling the electron loss to the upperelectrode such that the plasma density can be increased without the needto unduly increase the power to the plasma. By increasing the plasmadensity without a concomitant increase to the RF power requirement, thetarget layer can be etched at a higher rate without unduly degrading thephoto resist selectivity. In an embodiment, the upper electrode isconfigured such that the upper electrode is negatively biased, therebyallowing electrons present in the plasma chamber to be repelled from theupper electrode and trapped within the plasma volume for a longer periodof time. As the negatively charged electrons are trapped for a longerperiod of time, the plasma density is increased.

Generally speaking, during plasma processing the bombardment mechanismcauses electrons to be emitted from the substrate. As discussed earlier,electron loss to the upper electrode limits the increase in plasmadensity since the electron loss creates saturation point effect whichlimits the plasma density increase irrespective of the RF power providedto the plasma. By driving the upper electrode more negatively, theelectrons are thus repelled from the upper electrode instead of being,quickly lost to the upper electrode, resulting in a greater number ofelectrons in the plasma, thereby increasing the plasma density. Thehigher plasma density then can more effectively etch the target layer toachieve the desired high etch rate. Since it is unnecessary to increasethe RF power to achieve the high level of plasma density, photo resistselectivity is not adversely affected to the same degree as might havebeen in the prior art.

The above summary relates to only one of the many embodiments of theinvention disclosed herein and is not intended to limit the scope of theinvention, which is set forth in the claims herein. These and otherfeatures of the present invention will be described in more detail belowin the detailed description of the invention and in conjunction with thefollowing figures.

FIG. 3 shows, in accordance with an embodiment of the present invention,a simplified diagram of an implementation wherein a mirroring circuit isemployed to detect an RF signal from the lower electrode and to providethe upper electrode with a transformed RF signal that is in-phase withthe RF signal of lower electrode during plasma processing. As the termis employed herein, in-phase denotes the implementation wherein thephase difference between the RF signal to the lower electrode and the RFsignal to the upper electrode is within about 1%.

In the implementation of FIG. 3, lower electrode 304 is provided withmultiple RF frequencies signal 302 such as 2 Megahertz, 27 Megahertz,and 60 Megahertz. In an embodiment, the RF signal from lower electrode304 may be detected by probe 306, wherein probe 306 is a phase andamplitude detector designed to pick up low frequency RF signal, i.e.,frequencies less than 10 Megahertz.

In accordance with an embodiment of the present invention, the signalfrom the probe 306 is directed to a control circuit 308. The controlcircuit 308 is provided with the capability for phase and amplitudeadjustment allowing for the modification of the phase and/or amplitudeof the RF signal depending on whether the RF signal is to be in-phase orout-of-phase with the RF signal from the lower electrode 304.

The control signal coming out of control circuit 308 is directed to anRF signal generator 310 for generating an RF signal. Thereafter, the RFsignal generated by RF generator is optionally amplified (via amplifier320) to the desired phase or amplitude. In the context of the embodimentof the present invention, the amplitudes of the RF signals from theupper and lower electrodes are considered to be the same when the valuesof the amplitudes are within about 1% of each other.

In the implementation of FIG. 3, the amplified RF signal from theamplifier 320 is directed to the upper electrode 312. Consequently, theRF signal being directed to the upper electrode is in-phase with the RFsignal being supplied to the lower electrode in accordance with anembodiment of the present invention.

The features and advantages of having the RF signal directed to theupper electrode running in-phase with the RF signal from lower electrodein-phase can be better understood through FIGS. 4 a and 4 b. FIG. 4 ashows an example plot of an RF signal from the lower electrode, inaccordance with one embodiment of the present invention. FIG. 4 b showsan example plot of an RF signal directed to the upper electrode runningin phase with the RF signal from the lower electrode of FIG. 4 a, inaccordance with one embodiment of the present invention.

As mentioned previously, in-phase denotes the implementation wherein thephase difference between the RF signal to the lower electrode 304 andthe RF signal to the upper electrode 312 is within about 1%. At theminimal points during the negative cycles of the implementation of FIGS.4 a and 4 b, the RF signal 410 of the lower electrode and the RF signal450 of the upper electrode are at the most negative voltage values withrespect to the plasma.

Referring back to FIG. 3 when both RF signals are in-phase and at theirminimal, as shown in FIGS. 4 a and 4 b, during plasma processing in theplasma chamber 314, the upper electrode 312 and lower electrode 304 areat their most negative values. The positive charged argon particles (notshown) in the plasma chamber 314 will accelerate and bombard the upperelectrode 312 and the substrate 316, which is disposed above the lowerelectrode 304, to generate primary electrons which are low energyelectrons and secondary embedded electrons which are high energyelectrons.

Since both the substrate 316, disposed atop the lower electrode 304, andthe upper electrode 312, during this negative cycle of RF signals, areat their most negative values, the maximum potential between the upperelectrode 312 and lower electrode 304 with the plasma creates thehighest electron trapping. The electrons that come off of the upperelectrode 312 or the substrate 316 tend to be trapped between thenegatively biased upper electrode 312 and the negatively biasedsubstrate 316, which is disposed above the lower electrode 304. Sincethe electrons are negatively charged, the electrons might repel inbetween the two negatively charged upper electrode 312 and lowerelectrode 304. Instead of being immediately lost to upper electrode 312(as may be the case if upper electrode 312 is grounded, for example) thenegatively biased upper electrode 312 may repel the negatively chargedelectrons, thereby causing the electrons to be trapped in between upperelectrode 312 and lower electrode 304 for a longer period of time. It isbelieved that eventually, through the mechanism of random collision, thenegatively charged electrons are eventually lost to RF ground 318.

The longer residence time of the negatively charged electrons withinplasma chamber 314 contributes to a higher plasma density withoutrequiring a corresponding increase in the amount of RF power supplied toplasma processing chamber 300. Note that the mechanism to increase theplasma density of FIG. 3 does not require the increase in the RF powersupplied to the RF signals. Consequently, the photo resist selectivityis not negatively impacted to the same degree that might have beenimpacted had the higher plasma density been achieved by increasing theRF power level.

At the maxima points during the positive cycles of the implementation ofFIGS. 4 a and 4 b, the RF signal 420 of the lower electrode and the RFsignal 460 of the upper electrode are at the highest positive voltagevalues with respect to the plasma. In accordance with an embodiment ofthe present invention, secondary electrons are not being emitted duringthis time because the potential between the upper electrode 312 and thelower electrode 304 with respect to the plasma is low.

Further, during the positive cycle, the plasma potential in the plasmavolume is substantially higher than the potential of the peripheralground plate. It is believed that secondary electrons ejected from theseperipheral ground plates (e.g., ground plates 318 and 322) are alsotrapped in the plasma volume between the ground plates, resulting alsoin a longer residence time and a higher plasma density. Over the entirecycle (both negative and positive), the average plasma density is thusincreased.

In an embodiment, the phase difference between the RF signal from thelower electrode and the RF signal from the upper electrode can be usedas a knob to control the uniformity of etching, i.e., better photoresist selectivity to the underlying layer being etched. In theimplementation of FIG. 3, an arrangement where the phase of the RFsignal directed to the upper electrode 112 can be adjusted to the phaseof the RF signal of the lower electrode 304 during part of the cyclewhere the RF signals are at their most negative values.

For example, it is known that lower energy electrons and higher energyelectrons impact the etch process in different ways. Since a highdensity of higher energy electrons is believed to be beneficial forphoto resist selectivity, it is desirable in many cases to negativelybias upper electrode 312 to cause more of the higher energy electrons tobe trapped. It has been observed that unexpected beneficial etchinguniformity may be achieved by adjusting the phase difference between theRF signal directed to the upper electrode 312 and the RF signal from thelower electrode 304 during the negative cycle. In accordance with anembodiment of the present invention, the phase shifting is found to bebeneficial to etching uniformity for phase difference of less than about10%.

As can be appreciated from the foregoing, embodiments of the inventionachieve a higher level of plasma density to improve etching through thetarget layer in the capacitively-coupled plasma processing chamberwithout unduly damaging the photo resist during etching. By providing amechanism for increasing the plasma density without requiring aconcomitant increase in the RF power level of the RF signals provided tothe plasma processing chamber, plasma density is increased while PRphoto resist is maintained the same or is minimally impacted.Furthermore, the uniformity of etching is further enhanced through thecontrol of the phase difference between the RF signal directed to theupper electrode and the RF signal to the lower electrode.

In an embodiment, the phase of the upper electrode RF signal may beadjusted to either lag or lead the phase of the lower electrode RFsignal. When the upper electrode RF signal is out of phase with thelower electrode RF signal, it is observed that photoresist selectivityis reduced. For certain applications such as photoresist (PR) or polymerstrip, controlling the relative phases between the upper electrode RFsignal and lower electrode RF signal may improve the desired result ofremoving more PR or polymer.

Alternatively or additionally, the amplitude of the upper electrode RFsignal may be adjusted to either exceed or to be lower than theamplitude of the lower electrode RF signal. When the amplitude of theupper electrode RF signal is not equal to the amplitude of the lowerelectrode RF signal (defined herein as being different by more than 5%),it is observed that photoresist selectivity is reduced. As in the casewith the phase difference, for certain applications such as photoresist(PR) or polymer strip, controlling the relative amplitudes between theupper electrode RF signal and lower electrode RF signal may improve thedesired result of removing more PR or polymer.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents, whichfall within the scope of this invention. Also, the title, summary, andabstract are provided herein for convenience and should not be used toconstrue the scope of the claims herein. It should also be noted thatthere are many alternative ways of implementing the methods andapparatuses of the present invention. Although various examples areprovided herein, it is intended that these examples be illustrative andnot limiting with respect to the invention. It is therefore intendedthat the following appended claims be interpreted as including all suchalterations, permutations, and equivalents as fall within the truespirit and scope of the present invention.

What is claimed is:
 1. A method for processing a substrate in acapacitively-coupled plasma processing system, said capacitively-coupledplasma processing system having a plasma processing chamber and at leastan upper electrode and a lower electrode, said substrate being disposedon said lower electrode during plasma processing, comprising: providingat least a first RF signal from a first RF signal generator to saidlower electrode, said first RF signal having a first RF frequency, saidfirst RF signal coupling with a plasma in said plasma processingchamber, thereby inducing an induced RF signal on said upper electrode;providing a phase-mirroring arrangement to probe said lower electrode todetect said first RF signal and to adjust one of phase or amplitude ofthe first RF signal to define a control signal, the control signalprovided as an input to a second RF signal generator; providing a secondRF signal from said second RF signal generator to said upper electrode,said second RF signal also having said first RF frequency, a phase ofsaid second RF signal being offset from a phase of said first RF signalby a value that is less than 10%, wherein said phase and said firstfrequency of the second RF signal being generated based upon the inputfrom the control signal to the second RF signal generator withoutprobing said upper electrode; and processing said substrate while saidsecond RF signal is provided to said upper electrode, wherein the firstand second RF signal generators consist of separate RF signalgenerators.
 2. The method of claim 1, wherein said phase of said secondRF signal is in-phase with said phase of said first RF signal, asdefined by at least the control signal.
 3. The method of claim 1,wherein said second RF signal has an amplitude that is different from anamplitude of said first RF signal, as defined by at least the controlsignal.
 4. The method of claim 1, wherein said second RF signal has anamplitude that is the same as an amplitude of said first RF signal, asdefined by at least the control signal.
 5. The method of claim 1,wherein said first RF frequency is a low RF frequency signal.
 6. Themethod of claim 1, wherein said capacitively-coupled plasma processingsystem represents a multi-frequency capacitively-coupled plasmaprocessing system.
 7. The method of claim 1 wherein said phase of saidsecond RF signal is offset using the phase-mirroring arrangement.
 8. Themethod of claim 7, further comprising, amplifying the second RF signalproduced by the second RF signal generator.
 9. The method of claim 1,wherein the upper electrode is negatively biased.
 10. A method forprocessing a substrate in a capacitively-coupled plasma processingsystem, said capacitively-coupled plasma processing system having aplasma processing chamber and at least an upper electrode and a lowerelectrode, said substrate being disposed on said lower electrode duringplasma processing, comprising: providing said lower electrode at least afirst RF signal from a first RF signal generator, said first RF signalhaving a first RF frequency, said first RF signal coupling with a plasmain said plasma processing chamber, thereby inducing an induced RF signalon said upper electrode; and providing a phase-mirroring arrangementcoupled between said upper electrode and said lower electrode, saidphase-mirroring arrangement is configured to probe the lower electrodeto produce a control signal for generating a second RF signal from asecond RF signal generator to be directed to said upper electrode, saidsecond RF signal also having said first RF frequency, a phase of saidsecond RF signal being offset from a phase of said first RF signal by avalue that is less than 10%, wherein the control signal for generatingthe second RF signal that is directed to the upper electrode is notbased on probing of the upper electrode; said substrate is beingprocessed while said second RF signal is provided to said upperelectrode, wherein the first and second RF signal generators consist ofseparate RF signal generators.
 11. The method of claim 10, wherein saidphase of said second RF signal is in-phase with said phase of said firstRF signal, as set at least by the control signal.
 12. The method ofclaim 10, wherein said second RF signal has an amplitude that isdifferent from an amplitude of said first RF signal, as set at least bythe control signal.
 13. The method of claim 10, wherein said second RFsignal has an amplitude that is the same as an amplitude of said firstRF signal, as set at least by the control signal.
 14. The method ofclaim 10, wherein said first RF frequency is a low RF frequency signal.15. The method of claim 10, wherein said capacitively-coupled plasmaprocessing system represents a multi-frequency capacitively-coupledplasma processing system.
 16. The method of claim 10, furthercomprising, amplifying the second RF signal produced by the second RFsignal generator.
 17. The method of claim 10, wherein said phase of saidsecond RF signal is offset using a phase-mirroring arrangement.
 18. Themethod of claim 10, wherein the substrate is being processed while saidsecond RF signal is provided to said upper electrode configured with anegative bias.
 19. A method for processing a substrate in acapacitively-coupled plasma processing system, said capacitively-coupledplasma processing system having a plasma processing chamber and an upperelectrode and a lower electrode, said substrate being disposed on saidlower electrode during plasma processing, comprising: providing at leasta first RF signal from a first RF signal generator to said lowerelectrode, said first RF signal having a first RF frequency; probingsaid lower electrode to detect said first RF signal and to adjust one ofphase or amplitude of the first RF signal to define a control signal,the control signal provided as an input to a second RF signal generator;providing a second RF signal from said second RF signal generator tosaid upper electrode, said second RF signal also having said first RFfrequency, a phase of said second RF signal being offset from a phase ofsaid first RF signal by a value that is less than 10%, wherein saidphase and said first frequency of the second RF signal being generatedbased upon the input from the control signal to the second RF signalgenerator without probing said upper electrode; and processing saidsubstrate while said second RF signal is provided to said upperelectrode.
 20. The method of claim 19, wherein said phase of said secondRF signal is in-phase with said phase of said first RF signal, as set atleast by the control signal.